不同烧蚀条件下飞秒激光脉冲诱导ZnO纳米结构研究

烧蚀条件对飞秒激光脉冲诱导氧化锌纳米结构有重要影响。研究了800nm,150fs,250kHz的飞秒激光脉冲分别在空气中,去离子水中以及无水乙醇中垂直聚焦于氧化锌晶体表面,诱导形成不同形态的纳米结构。实验结果表明,在空气中利用飞秒激光脉冲辐照样品表面,形成了周期为180nm的纳米线;在去离子水中辐照诱导形成了由氧化锌纳米线聚集而成的"纳米球";在无水乙醇中形成出现分叉结构的纳米线。拉曼光谱分析辐照前后晶体晶相结果表明,形成的纳米结构相对于辐照前特征峰437cm-1强度有所下降,在570cm-1处的峰值则显著增强。分析了在各种烧蚀条件下诱导形成纳米结构的演化过程以及物理机理。

基于多台阶反射光栅的飞秒脉冲压缩装置

反射式光栅对是一种具有负色散性质的器件,可用于飞秒激光脉冲的压缩和展宽,具有无材料色散的优点。给出了一种基于多台阶反射光栅的脉冲压缩装置。该装置为倍密度光栅结构,由两个周期分别为40μm和20μm的四台阶反射式光栅组成。实验得到的衍射效率可以达到70%以上,输入脉冲经过两个光栅的衍射后会按原路返回,从而达到色散补偿的效果。利用此压缩装置,脉冲宽度为66.8 fs的输入脉冲压缩至接近傅里叶变换极限脉冲,即46.6 fs,由此证明只要多台阶光栅效率足够高,此装置就有可能成为不同于棱镜对进行飞秒脉冲腔内和腔外压缩的另一种途径。

Ultrafast temporal pattern generation and recognition

Ultrafast temporal pattern generation and recognition with femtosecond laser technology is presented, analyzed, and experimentally implemented. Ultrafast temporal pattern generation and recognition are realized by taking advantage of two well-known techniques: the space-time conversion technique and the ultrafast pulse measurement technique. Here the temporal pattern for the designed multiple pulses, optimized with a preassumed Gaussian spectral distribution of an ultrashort pulse, is described. With the simulation of a Gaussian spectral distribution, we realize that the uniformity of the generated multiple ultrafast temporal pulses is relevant to the repeated number of modulation periods in the mask in the spectral plane. Moreover, the change of Gaussian spectral phases with the wavelengths in the modulated phase plate is considered. Experiments of ultrafast temporal pattern recognition by the frequency-resolved optical gating (FROG) characterization technique are also given. (C) 2004 Society of Photo-Optical Instrumentation Engineers.

Experimental study of nonlinear switching characteristics of conventional 2×2 fused tapered couplers

The nonlinear switching characteristics of fused fiber directional couplers were studied experimentally. By using femtosecond laser pulses with pulse width of 100 fs and wavelength of about 1550 nm from a system of Ti:sapphire laser and optical parametric amplifier (OPA), the nonlinear switching properties of a null coupler and a 100% coupler were measured. The experimental results were coincident with the simulations based on nonlinear propagation equations in fiber by using super-mode theory. Nonlinear loss in fiber was also measured to get the injected power at the coupler. After deducting the nonlinear loss and input efficiency, the nonlinear switching critical peak powers for a 100% and a null fused couplers were calculated to be 9410 and 9440 W, respectively. The nonlinear loss parameter P_(N) in an expression of α_(NL)=αP/P_(N) was obtained to be P_(N)=0.23 W.

飞秒激光散斑相关法

利用飞秒激光散斑的信息，可以得知散射体的空间运动信息。为此提出了飞秒激光散斑相关测量技术，将散射体变化或移动前后的飞秒散斑场分别用CCD探测器记录下来并储存于计算机中，对移动前的飞秒散斑场和移动后的飞秒散斑场进行相关性运算，就可以确定出散射体的有关特性或移动量，实现对飞秒散斑场移动实时测量。结果表明，实验中散射体的位移量与用相关算法得到散射体的移动量相吻合，证实了飞秒散斑相关法测量的可行性。

Microoptical research in Shanghai Institute of Optics and Fine Mechanics

This paper summarized the recent research results of Changhe Zhou's group of Information Optics Lab in Shanghai Institute of Optics and Fine Mechanics (SIOM). The first is about the Talbot self-imaging research. We have found the symmetry rule, the regular-rearranged neighboring phase difference rule and the prime-number decamping rule, which is briefly summarized in a recent educational publication of Optics and Photonics News, pp.46-50, November 2004. The second is about four novel microoptical gratings designed and fabricated in SIOM. The third is about the design and fabrication of novel supperresolution phase plates for beam shaping and possible use in optical storage. The fourth is to develop novel femtosecond laser information processing techniques by incorporating microoptical elements, for example, use of a pair of reflective Dammann gratings for splitting the femtosecond laser pulses. The most attractive feature of this approach is that the conventional beam splitter is avoided. The conventional beam splitter would introduce the unequal dispersion due to the broadband spectrum of ultrashort laser pulses, which will affect the splitting result. We implemented the Dammann splitting apparatus by using two-layered reflective Dammann gratings, which generates the almost same array without angular dispersion. We believe that our device is highly interesting for splitting femtosecond laser pulses.

Experimental study of the FemtoTalbot effect

Experimental investigation of Talbot self-imaging effect of an amplitude grating under illumination of femtosecond laser pulse -- the FemtoTalbot effect is reported. Theoretical analyzed results show that Talbot images under illumination of femtosecond laser pulses are not the same as that under continuous wave illumination. Experimental results are in good agreement with the theoretical analysis. We believe that the experimental investigation of the FemtoTalbot effect is highly interesting for the enormous potential applications of Talbot effect.

频率分辨光学开关法测量飞秒脉冲

阐述了频率分辨光学开关法测量飞秒脉冲的原理，详细分析了模式尺寸效应和非线性效应对飞秒脉冲测量的影响。构建了一台用于飞秒脉冲测量的二次谐波-频率分辨光学开关装置，利用该装置对谐振腔输出的飞秒脉冲及压缩后的脉冲进行了测量。得到了飞秒脉冲的时间宽度及光谱宽度、电场及其相位在时域和频域的详细信息。谐振腔直接输出脉冲的时间宽度为56 fs，光谱宽度为27 nm，时间带宽积为0.686，算法中的最小误差为0.001792。脉冲压缩后的测量结果为27 fs，光谱宽度为92 nm，时间带宽积为1.27，算法误差为0.00

飞秒激光诱导波纹状微突起结构

利用飞秒激光振荡器产生的脉冲对镀有铬层的玻璃和石英基片进行微加工,发现两种样品表面均有波纹状的微突起结构产生。这些微突起结构离开样品表面的高度为10～300 nm不等,并且随着激光功率的增大而增加,在一定功率下达到饱和状态。它们的形貌、尺寸和高度取决于入射飞秒激光的能流以及飞秒脉冲的参数。通过化学方法证明了这些微突起结构是由玻璃和石英的主要成分SiO2组成的,并非样品表面的铬元素。此外,通过选取适当的飞秒激光功率和样品加工速度,制作了两种不同周期和线宽的光栅结构,显示出飞秒激光振荡器良好的加工性能。

基于反射式达曼光栅的频率分辨光学开关装置

采用反射式达曼光栅对飞秒激光进行分束,可以避免材料色散的影响。搭建了利用反射式1×2达曼光栅为基础的频率分辨光学开关(FROG)装置,并把测量结果与传统多发频率分辨光学开关装置的测量结果进行了对比。理论和实验结果表明,当输入脉冲宽度大于50 fs时,用达曼光栅作为分光器和使用分光镜分光的效果是一样的;当输入脉冲的宽度小于50 fs时,用达曼光栅作为分光器引入的展宽量明显小于分光镜引入的展宽量,尤其是当输入脉冲的宽度小于20 fs时用达曼光栅作为分光器的效果更为突出。

偏振光飞秒双脉冲微加工

利用具有纳焦能量、高重复频率的偏振光飞秒双脉冲对金属铬膜样品进行微加工,样品表面都会产生微突起状结构,它们的宽度在0～400 ps的双脉冲时延范围内没有明显的变化,但高度却都在1～10 ps的双脉冲时延范围内呈现明显的下降,在此时延范围之外并没有明显的变化。通过加工样品的扫描电子显微镜(SEM)图片发现,对于偏振光,利用双脉冲方法,可以获得更好的加工质量。并且线偏振光得到的微突起状结构比较细长,在入射光束的偏振方向上有所伸长;圆偏振光得到的微突起状结构比较接近圆形。即在低脉冲能量、高重复频率情况下,具体的微加工特征形貌与入射光束的偏振状态有关。

Generation of double pulses in-line by using reflective Dammann gratings

Doubled femtosecond laser pulses in-line are needed in the collinear pump-probe technique, collinear second harmonic generation frequency-resolved optical gating (SHG FROG) and the spectral phase interferometry for direct electric-field reconstruction (SPIDER), etc. Normally, it is generated by using a Michelson's structure. In this paper, we proposed a novel structure with two-layered reflective Dammann gratings and the reflective mirrors to generate doubled femtosecond laser pulses in line without transmission optical elements. Angular dispersion and spectral spatial walk-off are both compensated. In addition, this structure can also compress the positive chirped pulse, which cannot be realized with a Michelson's structure. By adopting triangular grating and blazed gratings, the efficiency of the system would in principle be increased as the Michelson's scheme. Experiments demonstrated that this method should be an alternative approach for generation of the double compressed pulses of femtosecond laser for practical applications. (c) 2006 Elsevier GmbH. All rights reserved.

Miniature pulse compressor of deep-etched gratings

We propose a miniature pulse compressor that can be used to compensate the group velocity dispersion that is produced by a commercial femtosecond laser cavity. The compressor is composed of two identical highly efficient deep-etched transmissive gratings. Compared with prism pairs, highly efficient deep-etched transmissive grating pairs are lightweight and small. With an optimized groove depth and a duty cycle, 98% diffraction efficiency of the -1 transmissive order can be achieved at a wavelength of 800 nm under Littrow conditions. The deep-etched gratings are fabricated in fused silica by inductively coupled plasma etching. With a pair of the fabricated gratings, the input positively chirped 73.9 fs pulses are neatly compressed into the nearly Fourier transform-limited 43.2 fs pulses. The miniature deep-etched grating-based pulse compressor should be of interest for practical applications. (c) 2008 Optical Society of America

飞秒激光辐射下微突起结构形成的超快热弹性模拟

本文结合有限元方法和超快热弹性模型对飞秒激光辐射下形成的微突起结构进行了数值模拟研究。模拟结果表明微突起结构的形成与入射飞秒激光的参数以及材料的热弹性质有关。在圆锥状微突起结构的形状和高度方面，实验结果与模拟结果呈现良好的一致性，这也从实验上表明了超快热弹性模型的有效性。本文的研究将有助于利用超快激光对薄膜材料进行纳米构造。

飞秒激光诱导铬膜产生周期性微结构

通过纳焦量级的飞秒激光在铬膜表面诱导出了周期性微结构,并使用入射飞秒激光和激发的表面等离子体波之间的干涉理论模拟分析了飞秒激光作用下铬膜表面的温度场分布情况,定性地解释了铬膜表面周期性微结构产生的机理。实验和理论结果有助于对飞秒激光和铬膜相互作用机制的理解。